Nowadays, the electronics for microelectronic semiconductor components are dominated by integrated circuits. These integrated circuits are formed from a complex arrangement of electronic structures which are connected to one another in a number of levels, arranged one above the other, on a semiconductor mount which is referred to as a chip. The joint production of chips on a semiconductor wafer is characterized by a complicated sequence of different process steps.
The main objective of the semiconductor industry is continuous performance improvement by means of ever faster circuits while at the same time further miniaturizing the electronic structures. In the course of this development process, there has been an increasing trend towards three-dimensional structures for the production of chips on semiconductor wafers, with the advantage of reducing the space required on the surfaces of the semiconductor wafers. In general, depressions or trenches which are formed in the semiconductor wafers act as the original structures for such three-dimensional structures, and are etched into the surfaces of the semiconductor wafers and/or into layers applied to the surfaces of the semiconductor wafers.
The miniaturization of the electronic structures is linked to more stringent requirements for the precision of the production processes that are used. At the same time, precision inspection methods are required in order to monitor the production processes. With regard to three-dimensional structures, accurate determination of the depth of depressions formed in the semiconductor wafers is, in particular, of major importance since this parameter can have a considerable influence on the functionality of the chips.
At present atomic force microscopes are preferably used to determine the depth of depressions formed in the semiconductor wafers, based on the use of a measurement probe to scan the surface to be investigated. The interaction processes (Van der Waal bonding forces) which occur as the probe mechanically approaches the surface, can thus be used to obtain surface information. This allows the depth of depressions to be measured with high accuracy of approximately 1 nanometer. Owing to the continuous miniaturization of the structures, the depressions to be measured are, however, becoming ever narrower, thus resulting in more stringent requirements for the geometry, the robustness and the resistance to wear of the measurement probes that are used, as well as more stringent requirements for the scanning process. This is particularly true of structures and depressions with a high aspect ratio, that is to say a high ratio of the depth to the lateral size. Depth determination by means of atomic force microscopes is thus becoming ever more costly and complex. Furthermore, the measurement times are increasing, thus reducing the throughput of measurable semiconductor wafers.
Alternatively, scanning electron microscopes can be used to determine the depth of depressions. In this case, the semiconductor wafer to be investigated is broken in the area of the depressions and a scanning electron microscope is used to record the fracture edge. However, this method is costly and tedious owing to the need to break the semiconductor wafer. Furthermore, the semiconductor wafer is destroyed by breaking it so that the method is highly costly and is suitable only for off-line measurement of only a small proportion of the semiconductor wafers.
3D scanning electron microscopes can be used for non-destructive determination of depths, in which a scanning electron microscope is used to record a surface and at least two different tilt angles. These records are then correlated with one another in order to calculate a 3D data record. This data record can be used to obtain cross-sectional information, and thus to determine the depth of depressions. The very long measurement times have been found to be a major disadvantage of this method. Furthermore, the depth determination becomes increasingly inaccurate as the aspect ratio of the depressions increases.
WO 02/03449 A2 discloses a method and a measurement apparatus for determination of the thickness of layers formed on semiconductor wafers. This is done by measuring the weight of the semiconductor wafer before and after the deposition of a layer, by means of a weighing device. The layer thickness or else the density of the layer can then be deduced from the difference between the recorded weight values. Even a large-area or structured deposited layer, deposited, for example, by means of an etching process, can be recorded by means of this differential weighing method. However, it has the disadvantage that two independent weight measurements are carried out, that is to say before and after the deposition process or etching process. Since, on the one hand, errors from these two independent measurements may be superimposed and, on the other hand, drift may occur in the weighing device between the two measurements, the accuracy of the measurement method is limited. Furthermore, only a representative mean value can be determined for the parameter of interest, such as the layer thickness, and this does not allow any statement to be made about its local distribution.